Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: coreysan on January 25, 2015, 08:22:03 pm
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I noticed on all Fender black face amps, the main ground wire from the filter caps goes up into the chassis, along the length of the chassis,
and meets up with 2 grounds from the power transformer.
Is that really necessary? In other words, why not just solder the filter cap ground wire to the underside of the chassis where the
caps are, so that the ground wire is only a few inches long, instead 10-12 inches?
I've heard rumors that the cap ground solder point should be near the PT. Is that a critical thing?
Thanks so much for any insight you can offer!
Corey
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I noticed on all Fender black face amps, the main ground wire from the filter caps goes up into the chassis, along the length of the chassis,
and meets up with 2 grounds from the power transformer.
The important thing is it meets with the PT grounds.
Specifically, the first filter cap should have the most-direct connection to the high voltage center tap possible. In Fender's case, that meant running the long wire to the same point that center-tap is grounded. This is because there are large current pulses when that first filter cap charges through the circuit made of that filter cap, the rectifier, the PT winding and its center-tap.
If you grounded that filter cap somewhere else on the chassis far from the center-tap ground, those big current pulses could induce hum in other circuit grounded between the filter cap and the center-tap.
The overall rule is connect the grounds of filter caps as directly as possible to the circuits they feed, including tube cathodes or cathode resistors. This is most-critical for the first filter cap, because the currents are larger than any other part of the amp.
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It's desirable to have EVERY wire in the amp as short as it can be; but that ground wire is the return path of every miliamp used by any DC-powered anything in the amp (eg; not the pilot lamp and not the tube heaters) Long experience has shown that grounding that wire any-old-anywhere is not the way to go. It wants to be grounded close to the power transformer, otherwise hum and ground-loop noise (also hum) can result. On a Princeton, this happens automatically because the can of the multi-section electrolytic is soldered to the chassis, providing an excellent ground (and mechanical mounting) right where that cap sits. Most tube amps want to see that "power supply ground" brought to a chassis point near the power trans, very typically to a lug tightened down under a transformer mounting bolt. This is what we call a "best practice"---a way of doing things that has proven beneficial over thousands of examples. The reason is avoid unpredictable currents through the metal of the chassis, which NEVER do you any good.
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Doug Hoffman's Library is full of useful info:
http://el34world.com/charts/grounds.htm (http://el34world.com/charts/grounds.htm)
You really have to stare at this one several times before it begins to connect - at least I did:
http://el34world.com/charts/currentflow.htm (http://el34world.com/charts/currentflow.htm)
It's very relevant to your question because of the current considerations HBP and eleventeen bring up. Note that the ground for the output transformer ground wire connected to the speaker jack(s) carries a LOT of current too.
Cheers,
Chip
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Here's a good read on grounding;
http://www.valvewizard.co.uk/Grounding.html (http://www.valvewizard.co.uk/Grounding.html)
Brad :icon_biggrin:
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You guys are geniuses. I really do appreciate your contribution. This is my first build, having fun,
and learning tons, thanks to you all!
Many, many thanks!
Corey